1. Technical Field
The invention concerns a method for producing lengthwise welded metal tubes in which a metal tape is drawn from a supply reel and is gradually formed into a slotted tube. The slotted tube is welded by a laser beam while the edges of the tape are preheated by a separate laser beam.
2. Description of the Prior Art
A method for producing lengthwise welded tubes is known from DE 4 117 263 A1. The known method is used to circumvent expensive tape degreasing by means of chlorinated hydrocarbons, which cause ecological concerns. To that end, the tape edge areas are heated before welding to such a temperature that grease, which adheres to the tape surface, evaporates. The grease coating on the tape surface occurs when the manufacturer rolls the sheet metal, and it remains on the surface to protect the metal against corrosion. The grease is often a hindrance during welding.
According to a first variation, the grease is evaporated by a laser beam, which also welds the tape edges. According to a second variation, the grease is evaporated by a laser beam and the welding is accomplished with an electric arc. The laser beam heats the tape edges to about 500.degree. C. so that all the grease evaporates, and the tape edges at the welding point become preheated with respect to the ambient temperature. For constructive reasons, the distance between the focal spot of the laser beam and the weld area created by the electric arc is rather large, so that heat produced by the laser beam in the tape edges can diffuse more or less quickly, depending on the heat conductivity of the metal tape and the production speed. For example, when copper or aluminum tapes are welded, the tape edges in the welding spot area are barely preheated, while noticeable heating of the tape edges can be found with alloyed steels, e.g. stainless steel, because of the low heat conductivity.